Replication of T4 DNA in vivo requires several proteins encoded by the phage DNA including the gene 43 DNA polymerase, the gene 32 DNA unwinding protein, the gene 44 and 62 DNA-dependent ATPase, the gene 45 protein, and the gene 41 single-stranded DNA-requiring nucleotidase. We have purified these proteins and have shown that they stimulate DNA synthesis in vitro with the T4 DNA polymerase with nicked duplex templates. We find that initiation of new chains on single-stranded templates requires an additional factor which is not present after infection with T4 mutants in gene 61 (DNA-delay phenotype). This initiation occurs on many single-stranded DNA's and requires ribonucleotide triphosphates. We are presently studying the mechanisms by which chains are initiated with circular single-stranded templates with these proteins, as well as the mechanisms by which they accelerate the elongation of the primer and unwind duplex templates.